Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, Functional Electrical Stimulation (FES) systems have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients. Furthermore, in recent investigations Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Occipital Nerve Stimulation (ONS), in which leads are implanted in the tissue over the occipital nerves, has shown promise as a treatment for various headaches, including migraine headaches, cluster headaches, and cervicogenic headaches.
These implantable neurostimulation systems typically include one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator (e.g., an implantable pulse generator (IPG)) implanted remotely from the stimulation site, but coupled either directly to the neurostimulation lead(s) or indirectly to the neurostimulation lead(s) via a lead extension. Thus, electrical pulses can be delivered from the neurostimulator to the neurostimulation leads to stimulate the tissue and provide the desired efficacious therapy to the patient. The neurostimulation system may further comprise a handheld patient programmer in the form of a remote control (RC) to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. A typical stimulation parameter set may include the electrodes that are acting as anodes or cathodes, as well as the amplitude, duration, and rate of the stimulation pulses.
Thus, the RC can be used to instruct the neurostimulator to generate electrical stimulation pulses in accordance with the selected stimulation parameters. Typically, the stimulation parameters programmed into the neurostimulator can be adjusted by manipulating controls on the RC to modify the electrical stimulation provided by the neurostimulator system to the patient. Thus, in accordance with the stimulation parameters programmed by the RC, electrical pulses can be delivered from the neurostimulator to the stimulation electrode(s) to stimulate or activate a volume of tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. The best stimulus parameter set will typically be one that delivers stimulation energy to the volume of tissue that must be stimulated in order to provide the therapeutic benefit (e.g., treatment of pain), while minimizing the volume of non-target tissue that is stimulated.
The IPG may be programmed by a clinician, for example, by using a clinician's programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon. Typically, the RC can only control the neurostimulator in a limited manner (e.g., by only selecting a program or adjusting the pulse amplitude or pulse width), whereas the CP can be used to control all of the stimulation parameters, including which electrodes are cathodes or anodes.
In the context of an SCS procedure, one or more stimulation leads are introduced through the patient's back into the epidural space, such that the electrodes carried by the leads are arranged in a desired pattern and spacing to create an electrode array. One type of commercially available stimulation leads is a percutaneous lead, which comprises a cylindrical body with ring electrodes, and can be introduced into contact with the affected spinal tissue through a Touhy-like needle, which passes through the skin, between the desired vertebrae, and into the epidural space above the dura layer. After proper placement of the neurostimulation leads at the target area of the spinal cord, the leads are anchored in place at an exit site to prevent movement of the neurostimulation leads.
To facilitate the location of the neurostimulator away from the exit point of the neurostimulation leads, lead extensions are sometimes used. The neurostimulation leads, or the lead extensions, are then connected to the IPG, which can then be operated to generate electrical pulses that are delivered, through the electrodes, to the targeted tissue, and in particular, the dorsal column and dorsal root fibers within the spinal cord. Intra-operatively (i.e., during the surgical procedure), the neurostimulator may be operated to test the effect of stimulation and adjust the parameters of the stimulation for optimal pain relief. A computer program, such as Bionic Navigator®, available from Boston Scientific Neuromodulation Corporation, can be incorporated in a clinician's programmer (CP) (briefly discussed above) to facilitate selection of the stimulation parameters. Any incisions are then closed to fully implant the system. Post-operatively (i.e., after the surgical procedure has been completed), a clinician can adjust the stimulation parameters using the computerized programming system to re-optimize the therapy.
After implantation of the neurostimulation leads, it may be desirable to electrically monitor the physiological environment in which the neurostimulation leads have been implanted in order to perform any one of various functions.
For example, the efficacy of SCS is related to the ability to stimulate the spinal cord tissue that innervates the region of pain experienced by the patient. Thus, the working clinical paradigm is that achievement of an effective result from SCS depends on the neurostimulation lead or leads being placed in a location (both longitudinal, lateral, and depth) relative to the spinal tissue, such that the electrical stimulation will treat the region of pain (i.e., the target of treatment). If a lead is not correctly positioned, it is possible that the patient will receive little or no benefit from an implanted SCS system. Thus, correct lead placement can mean the difference between effective and ineffective pain therapy, and as such, precise positioning of the leads proximal to the targets of stimulation is critical to the success of the therapy.
For example, multi-lead configurations, which enable more programming options for optimizing therapy, have been increasingly used in SCS applications. The use of multiple leads that are grouped together in close proximity to each other at one general region of the patient (e.g., side-by-side parallel leads along the spinal cord of the patient), increases the stimulation area and penetration depth (therefore coverage), as well as enables more combinations of anodic and cathodic electrodes for stimulation, such as transverse multipolar (bipolar, tripolar, or quadra-polar) stimulation, in addition to any longitudinal single lead configuration. Furthermore, with these lead configurations, current can be manipulated between leads medio-laterally to create the desired stimulation field. The resulting stimulation field is highly dependent on the relative position of the electrodes selected for stimulation.
Although the neurostimulation lead(s) may initially be correctly positioned relative to each other or relative to the stimulation target(s), the neurostimulation lead(s) are at risk of migration relative to each other and/or relative to the stimulation target(s). The neurostimulation lead(s) may migrate both acutely (e.g., during posture change or during activity/exercise) or chronically. In the context of SCS, the neurostimulation lead(s) may potentially migrate in three dimensions: rostro-caudally (along the axis of the spinal cord), medio-laterally (lateral to the spinal cord), and dorsal-ventrally (depth of the lead relative to the spinal cord). Notably, because the thickness of the cerebral spinal fluid (CSF) between the neurostimulation lead(s) and the spinal cord vary along the length spinal cord, migration of the neurostimulation lead(s) in the rostro-caudal direction may necessarily in the lead(s) being subjected to a different volume of CSF. Once the leads(s) migrate from their original position, a corrective action, such as surgical repositioning or electronic reprogramming of the neurostimulation leads may need to be performed relocate the stimulation to the targeted tissue region. Further details discussing the detection of lead migration by measuring electrical parameters, such as impedance, field potential, and evoked action potentials, are provided in U.S. Pat. Nos. 7,684,869, 7,853,330, and 8,401,665, which are expressly incorporated herein by reference.
As another example of a reason for electrically monitoring the physiological environment of the neurostimulation leads is that the coupling efficiency between the active electrodes and the targeted tissue region may change (either increase or decrease) as a result of inherent changes in the tissue characteristics typically caused by the tissue encapsulation process, which eventually surrounds the neurostimulation lead(s) with fibrous collagenous tissue (i.e., scar tissue) in an attempt to isolate the foreign materials of the neurostimulation lead(s). If the coupling efficiency decreases as a result of the tissue encapsulation process (or other processes), the intensity of the stimulation may be too low to provide effective therapy, whereas if the coupling efficiency increases as a result of the tissue encapsulation process (or other processes), the intensity of the stimulation may be too high and may overstimulate the targeted tissue region, inadvertently stimulate non-targeted tissue, and/or waste energy. Thus, knowledge of the coupling efficiency between the electrodes and the target tissue will allow the intensity of the stimulation to be adjusted to provide for a safe and efficacious level of therapy. In one preferred embodiment, the impedance between the electrodes and the target tissue is measured to determine the coupling efficiency, such that the amplitude of the stimulation can be automatically adjusted, as described in U.S. Pat. No. 7,742,823, which is expressly incorporated herein by reference.
In addition to tracking the coupling efficiency between the electrodes and the target tissue, it may be desirable to provide insight into the state of the encapsulation process (e.g., if the scar tissue has matured, is developing, is nascent, or even absent, etc.), thereby providing an indication of the stability of the neurostimulation lead(s). For example, if the encapsulation process is in the early stages, the activity of the patient may be limited so that the encapsulation process is not disrupted. In contrast, if the encapsulation process is complete, the neurostimulation lead(s) may be stabilized, and thus, no physical limitations may be placed on the patient.
As still another example of a reason for electrically monitoring the physiological environment of the neurostimulation leads is that it may be desirable to track the physical activity (e.g., activity level or body manipulations) of the patient that has received the implantable neurostimulation system, which provides an indication of the efficacy of the therapy provided by the stimulation system; that is, the more efficacious the therapy, the more diurnally active the patient will be. Thus, knowledge of the physical activity of the patient over a period of time in which therapeutic stimulation is applied to the patient may be used by a physician or clinician to prescribe pharmaceuticals, reprogram or upgrade the IPG, or implement or modify other therapeutic regimens (such as physical or occupational therapy). Knowledge of the physical activity of the patient may also be used to adapt the therapy provided by the stimulation system in real time, so that the stimulation is consistently provided to the patient at an efficacious and/or comfortable level. Further details discussing the tracking of the physical activity of a patient are provided in U.S. patent application Ser. No. 12/024,947, entitled “Neurostimulation System and Method for Measuring Patient Activity,” which is expressly incorporated herein by reference.
There remains a need to provide improved techniques for characterizing the tissue surrounding a medical lead.